In recent years, rapid development of wireless communication technology has promoted the progress of the society, and has profoundly influenced and changed people's ways of production and life. Meanwhile, people's requirements for better mobile communication services have prompted the rapid development of wireless communication technology. The fourth generation (4G) mobile communication system is born and developed in the background. In order to achieve the performance required for the 4G mobile communication system, 3GPP has introduced many new technologies in the Long Term Evolution (LTE)-Advanced system evolution technology as one of the IMT-Advanced system technical standards, such as Relay technology, Carrier Aggregation (CA) technology, Coordinated Multiple Points (CoMP) transmission technology and so on. For the purpose of enhancing hotspot coverage, reducing dead zones of a network, improving the performance of user services at the edge of a network, and increasing the system capacity of the network, micro cells are introduced.
In a heterogeneous network, a Main eNodeB (MeNB) and a Secondary eNodeB (SeNB) have different transmit power, and path losses from a user to the two eNBs are also different. Therefore, in consideration of the performance of uplink and downlink throughput for the user, optimal access cells of the user are different. In the LTE, as the user may implement cell selection or cell reselection according to Reference Signal Received Power (RSRP), the user may access a cell with the strongest received signal. This means that the user cannot access the optimal cells in the uplink and downlink at the same time. In this case, the optimal solution should be that the user accesses the optimal cells in the uplink and downlink respectively.
In the 3GPP TR 36.839, switching performance of a user is worse in the heterogeneous network than in a pure macro cell network, especially when a micro cell and a macro cell are deployed in the same frequency. In order to avoid such problem and ensure robustness of user mobility while enhancing load balancing, a solution is that the micro cell and the macro cell provide RRC signaling related to switching at the same time.
It has been mentioned in the 3GPP TR 36.839 that a switching frequency is much higher in a micro cell scenario than in a macro cell scenario, and when cell switching is performed at a high frequency, it will increase signaling overhead of the access network and the core network. In the current LTE architecture, a control plane bearer and a user plane bearer of a user end at the same eNB. When the user performs switching from a micro cell to another micro cell, it needs to change the control plane bearer and the user plane bearer of the user. In order to solve such problem, the user may access multiple eNBs at the same time, for example, the control plane bearer of the user is connected to the MeNB all the time, and the user plane bearer may be connected to the micro cell.
In the Release-10 and the Release-11, the carrier aggregation may largely enhance a peak rate of a user. However, due to the limitation by the delay, the carrier aggregation can only be completed within the same eNB. Therefore, in the heterogeneous network, carrier aggregation across eNBs cannot be used currently, which means the peak rate of the user is lower in the heterogeneous network than in the carrier aggregation scenario. In this case, if the user can access multiple eNBs at the same time, a higher peak rate can be acquired and cell load can be optimized.
A potential solution is to access the macro cell and the micro cell at the same time. The technical solution of a user accessing the macro cell and the micro cell at the same time is referred to as dual connectivity in the Release-12. The dual connectivity may be defined to be that a user may access multiple eNBs in the same frequency or different frequencies at the same time. In the 3GPP TR 36.932, a working scenario may be that there is an ideal or non-ideal backhaul link between the eNBs. For a scenario that there is an ideal backhaul link between the eNBs, related technologies such as CA, CoMP or the like in the Release-10/11 may be used. Therefore, in the Release-12, a working scenario of a non-ideal backhaul link is primarily concerned.
Control of uplink power of a transmitter plays a very important role in the mobile communication system, and is used to achieve a balance between two requirements below. One requirement is to achieve sufficient transmitted energy per bit required for Quality of Service (QoS), and the other requirement is to minimize the interference to other users of the system and maximize the battery life of the mobile terminal. In order to achieve this purpose, the control of the uplink power must adapt to the characteristics of the wireless propagation channel, including a path loss, a shadow effect, and fast fading, and overcome the interference from other users of the same cell and adjacent cells.
A process of controlling uplink power in the LTE is primarily as follows.
(1) A UE receives a Transmit Power Control (TPC) command.
(2) The UE measures pilot received power of a downlink wireless channel and calculates a path loss. The UE calculates transmit power P based on parameters such as the path loss, a bandwidth of an uplink shared channel, a transport format and the TPC or the like. If the UE triggers a Power Headroom Report (PHR) and a transmission condition is satisfied, the UE further needs to transmit the PHR on the uplink shared channel.
(3) After receiving the PHR on the physical uplink shared channel, the eNB transmits the TPC to the UE through a Physical Downlink Control CHannel (PDCCH) in a format of DCI Format 0/1/1A/1B/1D/2/2A/2B/2C/2D/3/3N4.
(4) Return to step (1), i.e., the UE receives a TPC command.
In steps (2) and (3) described above, the PHR transmitted by the UE to the eNB, as resource budget information of a Physical Uplink Shared CHannel (PUSCH), provides the eNB with a basis for allocating uplink resources. If the PHR reflects that the UE has large power headroom, the eNB may allocate more wireless resources to the UE; and if the PHR reflects that the UE has no or a few power headroom, the eNB can only allocate a few wireless resource blocks or a low-order modulation order to the UE. The PHR needs to be transmitted in two steps, i.e., triggering and then reporting. The UE triggers report of the PHR when a related timer expires, or a change in the path loss is large, or the PHR is configured/reconfigured, or the cell is activated or deactivated, and the UE reports the PHR to the eNB after acquiring enough PUSCH resources. The PHR, as a Media Access Control Control Element (MAC CE), is multiplexed in a MAC Protocol Data Unit (PDU), and is transmitted through the PUSCH.
PCMAX is configured maximum transmit power of the UE. A value range of PCMAX is primarily limited by two factors, which is limitation by the control of the eNB, and useless out-band radiation of the UE. For example, in order to satisfy the requirements for Spectrum Emission Mask (SEM) and Adjacent Channel Leakage Ratio (ACLR) or the like, the UE reduces the maximum transmit power (also referred to as power back-off).
In conclusion, as the new technology of dual connectivity is introduced in the Release-12, the UE can access the macro cell and the micro cell at the same time. It should be illustrated that the macro cell and the micro cell discussed above should be construed as a macro eNB and a micro eNB, and therefore, a cell here should be construed as an eNB. As there is no ideal backhaul link between the eNBs, the UE needs to transmit an uplink signal (an (ACK/NACK, CSI information, a UL-SCH or the like) to the macro cell/eNB and the micro cell/eNB respectively. At the same time, as the macro cell/eNB and the micro cell/eNB independently schedule the UE respectively, this will results in a dual connectivity UE does not have enough uplink transmit power to support simultaneous uplink transmission to both the macro cell/eNB and the micro cell/eNB. Obviously, the mechanism of controlling uplink power in the current Release-10/11 cannot solve the current problem. In the present disclosure, the cells/eNBs accessed by the UE are divided into a Primary Cell (PCell)/eNB and a Secondary Cell (SCell)/eNB respectively below. Obviously, after the carrier aggregation technology is used, various eNBs comprise multiple cells for a user, and therefore, the MeNB and the SeNB may be referred to as a group of PCells and a group of SCells.